Systems and methods for current control of power conversion systems

ABSTRACT

System and method for regulating an output current of a power conversion system. An example system controller for regulating an output current of a power conversion system includes a driving component, a demagnetization detector, a current-regulation component, and a signal processing component. The driving component is configured to output a drive signal to a switch in order to affect a primary current flowing through a primary winding of the power conversion system. The demagnetization detector is configured to receive a feedback signal associated with an output voltage of the power conversion system and generate a detection signal based on at least information associated with the feedback signal. The current-regulation component is configured to receive the drive signal, the detection signal and a current-sensing signal and output a current-regulation signal based on at least information associated with the drive signal, the detection signal, and the current sensing signal.

1. CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201210258359.X, filed Jul. 24, 2012, commonly assigned, incorporated byreference herein for all purposes.

2. BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for currentcontrol. Merely by way of example, the invention has been applied toconstant current control of power conversion systems. But it would berecognized that the invention has a much broader range of applicability.

FIG. 1 is a simplified diagram showing a conventional flyback powerconversion system. The power conversion system 100 includes a primarywinding 110, a secondary winding 112, an auxiliary winding 114, a powerswitch 120, a current sensing resistor 130, two rectifying diodes 160and 168, two capacitors 162 and 170, and two resistors 164 and 166. Forexample, the power switch 120 is a bipolar transistor. In anotherexample, the power switch 120 is a metal-oxide-semiconductor (MOS)transistor. In yet another example, the power conversion system 100provides power to one or more light-emitting-diodes (LED) 199.

The auxiliary winding 114 can be used to extract information associatedwith an output voltage 150 on the secondary side so that the outputvoltage 150 can be regulated. When the power switch 120 is closed (e.g.,on), the energy is stored in the transformer including the primarywinding 110 and the secondary winding 112. When the power switch 120 isopen (e.g., off), the stored energy is released to the output terminal.The voltage of the auxiliary winding 114 maps the output voltage 150.The auxiliary winding 114 and associated components (e.g., the resistors164 and 166) generates a feedback signal 174 which can be determinedbased on the following equation:

$\begin{matrix}{V_{FB} = {\frac{R_{2}}{R_{1} + R_{2}} \times V_{aux}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where V_(FB) represents the feedback signal 174, and V_(aux) representsa voltage 154 of the auxiliary winding 114. R₁ and R₂ represent theresistance values of the resistors 164 and 166 respectively.

For example, the switch 120 is associated with a switching periodincluding an on-time period during which the switch 120 is closed (e.g.,on) and an off-time period during which the switch 120 is open (e.g.,off). As an example, in a continuous conduction mode (CCM), a nextswitching cycle starts prior to the completion of a demagnetizationprocess associated with the transformer including the primary winding110 and the secondary winding 112. Therefore, the duration of thedemagnetization process (e.g., a demagnetization period) before the nextswitching cycle starts is approximately equal to the off-time period ofthe switch. For example, in a discontinuous conduction mode (DCM), anext switching cycle does not start until a time period after thedemagnetization process has completed. In another example, in a criticalconduction mode (CRM) (e.g., a quasi-resonant (QR) mode), a nextswitching cycle starts shortly after the completion of thedemagnetization process.

FIG. 2 is a simplified conventional timing diagram for the flyback powerconversion system 100 that operates in the continuous conduction mode(CCM). The waveform 202 represents the feedback signal 174 of theauxiliary winding 114 as a function of time, the waveform 204 representsa primary current 176 that flows through the primary winding 110 as afunction of time, and the waveform 206 represents a secondary current178 that flows through the secondary winding 112 as a function of time.

For example, a switching period, T_(s), starts at time t₀ and ends attime t₂, an on-time period, T_(on), starts at the time t₀ and ends attime t₁, and a demagnetization period, T_(dem), starts at the time t₁and ends at the time t₂. In another example, an off-time period isapproximately equal in duration to the demagnetization period. In yetanother example, t₀≦t₁≦t₂.

During the on-time period T_(on), the power switch 120 is closed (e.g.,on), and the primary current 176 flows through the primary winding 110and increases from a magnitude 208 (e.g., at t₀) to a magnitude 210(e.g., at t₁) as shown by the waveform 204. The secondary current 178 isat a low magnitude 212 (e.g., approximately zero) as shown by thewaveform 206. The feedback signal 174 keeps at a magnitude 214 (e.g., asshown by the waveform 202).

At the beginning of the demagnetization period T_(off) (e.g., at t₁),the switch 120 is open (e.g., off), the primary current 176 is reducedfrom the magnitude 210 to a magnitude 216 (e.g., approximately zero) asshown by the waveform 204. The secondary current 178 increases from themagnitude 212 (e.g., approximately zero) to a magnitude 218 as shown bythe waveform 206. The feedback signal 174 increases from the magnitude214 to a magnitude 220 (e.g., as shown by the waveform 202).

During the demagnetization period T_(dem), the switch 120 remains open,the primary current 176 keeps at the magnitude 216 (e.g., approximatelyzero) as shown by the waveform 204. The secondary current 178 decreasesfrom the magnitude 218 (e.g., at t₁) to a magnitude 222 (e.g., at t₂) asshown by the waveform 206. The feedback signal 174 decreases from themagnitude 220 to a magnitude 222 (e.g., as shown by the waveform 202).

At the end of the demagnetization period T_(dem) (e.g., at t₂), a nextswitching cycle starts before the demagnetization process is completed.The residual energy reflects back to the primary winding 110 and appearsas an initial primary current 224 at the beginning of the next switchingcycle.

As an example, the primary current 176 and the secondary current 178satisfy the following equations:I _(sec) _(—) ₁ =N×I _(pri) _(—) ₁  (Equation 2)I _(sec) _(—) ₀ =N×I _(pri) _(—) ₀  (Equation 3)where I_(sce) _(—) ₁ represents the secondary current 178 when thedemagnetization period T_(dem) starts, and I_(sec) _(—) ₀ represents thesecondary current 178 when the demagnetization period T_(dem) ends.Additionally, I_(pri) _(—) ₁ represents the primary current 176 when theon-time period T_(on) ends, I_(pri) _(—) ₀ represents the primarycurrent 176 when the on-time period T_(on) starts, and N represents aturns ratio between the primary winding 110 and the secondary winding112.

For example, the output current 152 is equal to an average of thesecondary current 178 as shown by the following equation.

$\begin{matrix}{I_{out} = {\frac{1}{2} \times \frac{1}{T} \times {\int_{0}^{T}{\left( {I_{\sec\;\_ 1} + I_{\sec\;\_ 0}} \right) \times \frac{T_{dem}}{T_{s}}\ {\mathbb{d}t}}}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$where I_(out) represents an output current 152 on the secondary side, Trepresents an integration period, T_(s) represents a switching period,and T_(dem) represents the duration of the demagnetization processwithin the switching period.

Therefore, the output current 152 satisfies the following equation:

$\begin{matrix}{I_{out} = {\frac{N}{2} \times \frac{1}{T} \times {\int_{0}^{T}{\left( {I_{{pri}\;\_ 1} + I_{{pri}\;\_ 0}} \right) \times \frac{T_{dem}}{T_{s}}\ {\mathbb{d}t}}}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

As shown in FIG. 1, the resistor 130, in combination with othercomponents, generates a current-sensing voltage signal 172 which isrelated to the primary current 176. For example, the output current 152can be determined according to the following equations:

$\begin{matrix}{\mspace{79mu}{{I_{out} = {\frac{N}{2} \times \frac{1}{R_{s}} \times \frac{1}{T} \times {\int_{0}^{T}{\left( {V_{{cs}\; 1} + V_{{cs}\; 0}} \right) \times \frac{T_{dem}}{T_{s}}\ {\mathbb{d}t}}}}}\mspace{79mu}{or}}} & \left( {{Equation}\mspace{14mu} 6} \right) \\{I_{out} = {\frac{N}{2} \times \frac{1}{R_{s}} \times \frac{1}{K} \times {\sum\limits_{1}^{K}{\left( {{V_{{cs}\; 1}(n)} + {V_{{cs}\; 0}(n)}} \right) \times \frac{T_{dem}(n)}{T_{s}(n)}}}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$where V_(cs0) represents the current-sensing voltage signal 172 when anon-time period starts during a switching cycle, V_(cs1) represents thecurrent-sensing voltage signal 172 when the on-time period ends duringthe switching cycle, and R_(s) represents the resistance of the resistor130. In addition, n corresponds to the n^(th) switching cycle,V_(cs0)(n) represents a magnitude of the current-sensing voltage signal172 when an on-time period T_(on) starts in the n^(th) switching cycle,V_(cs1)(n) represents a magnitude of the current-sensing voltage signal172 when the on-time period ends in the n^(th) switching cycle, and Krepresents the number of switching cycles that are included in thecalculation. For example, K can be infinite; that is, the calculation ofEquation 7 can include as many switching cycles as needed.

The output current 152 may thus be regulated based on informationassociated with the current-sensing voltage signal 172, thedemagnetization process, and/or the switching period. However, theconventional current control schemes often suffer from low measurementaccuracy.

Hence it is highly desirable to improve the techniques for currentcontrol of power conversion systems.

3. BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for currentcontrol. Merely by way of example, the invention has been applied toconstant current control of power conversion systems. But it would berecognized that the invention has a much broader range of applicability.

According to one embodiment, a system controller for regulating anoutput current of a power conversion system includes a drivingcomponent, a demagnetization detector, a current-regulation componentand a signal processing component. The driving component is configuredto output a drive signal to a switch in order to affect a primarycurrent flowing through a primary winding of the power conversionsystem, the drive signal being associated with at least a switchingperiod, the switching period including an on-time period and ademagnetization period. The demagnetization detector is configured toreceive a feedback signal associated with an output voltage of the powerconversion system and generate a detection signal based on at leastinformation associated with the feedback signal, the detection signalindicating the demagnetization period. The current-regulation componentis configured to receive the drive signal, the detection signal and acurrent-sensing signal and output a current-regulation signal based onat least information associated with the drive signal, the detectionsignal, and the current sensing signal, the current-sensing signalrepresenting the primary current in magnitude. In addition, the signalprocessing component is configured to receive the current-regulationsignal and output a processed signal to the driving component in orderto generate the drive signal.

According to another embodiment, a system controller for regulating anoutput current of a power conversion system includes a drivingcomponent, a current-regulation component, an amplifier and acomparator. The driving component is configured to output a drive signalto a switch in order to affect a primary current flowing through aprimary winding of the power conversion system, the drive signal beingassociated with at least a switching period, the switching periodincluding an on-time period and a demagnetization period. Thecurrent-regulation controller is configured to receive the drive signal,a detection signal and a current-sensing signal and output a firstsignal based on at least information associated with the drive signal,the detection signal, and the current sensing signal, the detectionsignal indicating the demagnetization period, the current-sensing signalrepresenting the primary current in magnitude. The amplifier isconfigured to receive the first signal and a reference signal andgenerate, with at least a capacitor, an amplified signal based on atleast information associated with the first signal and the referencesignal. The comparator configured to receive the amplified signal and aramp signal and generate a comparison signal based on at leastinformation associated with the amplified signal and the ramp signal,the ramp signal being associated with at least a ramping period. Thedriving component is further configured to receive the comparison signaland a second signal associated with at least a signal period, processinformation associated with the comparison signal and the second signal,and generate the drive signal based on at least information associatedwith the comparison signal and the second signal.

According to yet another embodiment, a system controller for detecting ademagnetization period associated with a power conversion systemincludes a differentiation component, a comparator, and a detectioncomponent. The differentiation component is configured to receive afeedback signal associated with an output signal of the power conversionsystem and output a processed signal based on at least informationassociated with the feedback signal. The comparator is configured toreceive at least the processed signal and generate a comparison signalbased on at least information associated with the processed signal. Thedetection component is configured to receive at least the comparisonsignal and output a detection signal based on at least informationassociated with the comparison signal. The differentiation componentincludes a capacitor, a resistor and a current source. The capacitorincludes a first capacitor terminal and a second capacitor terminal, thefirst capacitor terminal being configured to receive the feedbacksignal. The resistor includes a first resistor terminal and a secondresistor terminal, the first resistor terminal being configured tooutput the processed signal, the second resistor terminal being biasedto a predetermined voltage. The current source includes a firstcomponent terminal and a second component terminal, the first componentterminal being coupled to the second capacitor terminal and the firstresistor terminal.

In one embodiment, a method for regulating an output current of a powerconversion system includes outputting a drive signal to a switch inorder to affect a primary current flowing through a primary winding ofthe power conversion system, the drive signal being associated with atleast a switching period, the switching period including an on-timeperiod and a demagnetization period, receiving a feedback signalassociated with an output voltage of the power conversion system, andprocessing information associated with the feedback signal. The methodfurther includes generating a detection signal based on at leastinformation associated with the feedback signal, the detection signalindicating the demagnetization period, receiving the drive signal, thedetection signal and a current-sensing signal, the current-sensingsignal representing the primary current in magnitude, and processinginformation associated with the drive signal, the detection signal andthe current-sensing signal. In addition, the method includes outputtinga current-regulation signal based on at least information associatedwith the drive signal, the detection signal, and the current sensingsignal, receiving the current-regulation signal, processing informationassociated with the current-regulation signal, and outputting aprocessed signal to the driving component in order to generate the drivesignal.

In another embodiment, a method for regulating an output current of apower conversion system includes outputting a drive signal to a switchin order to affect a primary current flowing through a primary windingof the power conversion system, the drive signal being associated withat least a switching period, the switching period including an on-timeperiod and a demagnetization period, receiving the drive signal, adetection signal and a current-sensing signal, the detection signalindicating the demagnetization period, the current-sensing signalrepresenting the primary current in magnitude, and processinginformation associated with the drive signal, the detection signal andthe current-sensing signal. The method further includes outputting afirst signal based on at least information associated with the drivesignal, the detection signal, and the current sensing signal, receivingthe first signal and a reference signal, and processing informationassociated with the first signal and the reference signal. In addition,the method includes generating, with at least a capacitor, an amplifiedsignal based on at least information associated with the first signaland the reference signal, receiving the amplified signal and a rampsignal, the ramp signal being associated with at least a ramping period,and processing information associated with the amplified signal and theramp signal. Furthermore, the method includes generating a comparisonsignal based on at least information associated with the amplifiedsignal and the ramp signal, receiving the comparison signal and a secondsignal associated with at least a signal period, processing informationassociated with the comparison signal and the second signal, andgenerating the drive signal based on at least information associatedwith the comparison signal and the second signal.

In yet another embodiment, a method for detecting a demagnetizationperiod associated with a power conversion system includes receiving, byat least a capacitor, a feedback signal associated with an output signalof the power conversion system, the capacitor including a firstcapacitor terminal and a second capacitor terminal, the first capacitorterminal receiving the feedback signal, the second capacitor terminalbeing coupled to a first component terminal of a current source, thecurrent source further including a second component terminal, providinga current, by the current source, to at least a resistor including afirst resistor terminal and a second resistor terminal, the firstresistor terminal being coupled to the first component terminal, thesecond resistor terminal being biased to a predetermined voltage, andprocessing information associated with the feedback signal and thecurrent. The method further includes outputting, by at least the firstresistor terminal, a processed signal based on at least informationassociated with the feedback signal and the current, receiving at leastthe processed signal and a reference signal, and processing informationassociated with the processed signal and the reference signal. Inaddition, the method includes generating a comparison signal based on atleast information associated with the processed signal and the referencesignal, receiving at least the comparison signal, and generating adetection signal based on at least information associated with thecomparison signal.

Many benefits are achieved by way of the present invention overconventional techniques. For example, some embodiments of the presentinvention sample a current sensing signal at a middle point of anon-time period to reduce the sampling errors associated with samplingtwice the current sensing signal during the on-time period. In anotherexample, certain embodiments of the present invention sample the currentsensing signal at the middle point of an on-time period to reduce thesampling errors associated with sampling the current sensing signalshortly after the beginning of the on-time period. In yet anotherexample, some embodiments of the present invention accurately detect theend of a demagnetization process without including a time period relatedto resonance into the demagnetization period to reduce measurementerrors thereof.

Depending upon embodiment, one or more benefits may be achieved. Thesebenefits and various additional objects, features and advantages of thepresent invention can be fully appreciated with reference to thedetailed description and accompanying drawings that follow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing a conventional flyback powerconversion system.

FIG. 2 is a simplified conventional timing diagram for the flyback powerconversion system as shown in FIG. 1 that operates in the continuousconduction mode (CCM).

FIG. 3(A) is a simplified diagram showing a power conversion systemconfigured to operate in the continuous conduction mode (CCM) and/or inthe discontinuous conduction mode (DCM) according to an embodiment ofthe present invention.

FIG. 3(B) is a simplified diagram showing a power conversion systemconfigured to operate in the critical conduction mode (CRM) according toanother embodiment of the present invention.

FIG. 4 is a simplified diagram showing certain components of thecurrent-control component as part of the controller as shown in FIG.3(A) or as part of the controller as shown in FIG. 3(B) according tocertain embodiments of the present invention.

FIG. 5(A) is a simplified timing diagram for the controller as part ofthe power conversion system as shown in FIG. 3(A) operating in the CCMmode according to one embodiment of the present invention.

FIG. 5(B) is a simplified timing diagram for the controller as part ofthe power conversion system as shown in FIG. 3(A) operating in the DCMmode according to another embodiment of the present invention.

FIG. 5(C) is a simplified timing diagram for the controller as part ofthe power conversion system as shown in FIG. 3(B) operating in the CRMmode according to an embodiment of the present invention.

FIG. 6 is a simplified diagram showing certain components of thesampling component as part of the current-control component of thecontroller as shown in FIG. 3(A) or the controller as shown in FIG. 3(B)according to certain embodiments of the present invention.

FIG. 7 is a simplified timing diagram for the sampling component as partof the current-control component of the controller as shown in FIG. 3(A)or the controller as shown in FIG. 3(B) according to some embodiments ofthe present invention.

FIG. 8(A) is a simplified diagram showing certain components of thedemagnetization detector as part of the controller as shown in FIG. 3(A)or the controller as shown in FIG. 3(B) according to certain embodimentsof the present invention.

FIG. 8(B) is a simplified diagram showing certain components of thedemagnetization detector as part of the controller as shown in FIG. 3(A)or the controller as shown in FIG. 3(B) according to some embodiments ofthe present invention.

FIG. 9 is a simplified timing diagram for the demagnetization detectoras part of the controller as shown in FIG. 3(A) operating in the DCMmode according to certain embodiments of the present invention.

FIG. 10 is a simplified timing diagram for the demagnetization detectoras part of the controller as shown in FIG. 3(A) operating in the CCMmode according to some embodiments of the present invention.

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for currentcontrol. Merely by way of example, the invention has been applied toconstant current control of power conversion systems. But it would berecognized that the invention has a much broader range of applicability.

FIG. 3(A) is a simplified diagram showing a power conversion systemconfigured to operate in the continuous conduction mode (CCM) and/or inthe discontinuous conduction mode (DCM) according to an embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications.The power conversion system 300 includes a controller 302, a primarywinding 304, a secondary winding 306, a current sensing resistor 308, anauxiliary winding 310, three capacitors 312, 314 and 328, two resistors318 and 320, two rectifying diodes 322 and 324, and a power switch 316.The controller 302 includes a current-control component 326, anamplifier 330, a comparator 332, an oscillator 334, a flip-flopcomponent 336, a demagnetization detector 338, an AND gate 340, and adriving component 342. Further, the controller 302 includes terminals344, 346, 348, and 350. For example, the power switch 316 is a bipolartransistor. In another example, the power switch 316 is a field effecttransistor (e.g., a MOSFET).

According to one embodiment, when the power switch 316 is closed (e.g.,on), the energy is stored in the transformer including the primarywinding 304 and the secondary winding 306. For example, when the powerswitch 316 is open (e.g., off), the stored energy is released to theoutput terminal. In another example, the auxiliary winding 310 andassociated components (e.g., the resistors 318 and 320) generates afeedback signal 360 which is related to an output voltage 359 on thesecondary side. In yet another example, the demagnetization detector 338receives the feedback signal 360 and outputs a detection signal 356which indicates when a demagnetization process associated with thetransformer including the primary winding 304 and the secondary winding306 starts and when the demagnetization process ends. In yet anotherexample, the current-control component 326 receives the detection signal356, a current-sensing signal 354 generated by the resistor 308 and adrive signal 358 generated by the driving component 342, and outputs asignal 362 (e.g., V_(C)) to the amplifier 330. In yet another example,the amplifier 330 and the capacitor 328 are included in an integratorwhich integrates a difference between the signal 362 and a referencesignal 367 and outputs a signal 364. In yet another example, thedemagnetization detector 338 also receives the drive signal 358.

According to another embodiment, the comparator 332 receives the signal364 and a signal 366 generated by the oscillator 334, and outputs acomparison signal 368. For example, if the signal 366 (e.g., a rampsignal) is larger than the signal 364 in magnitude, the comparator 332generates the comparison signal 368 at a logic low level (e.g., 0). Inanother example, if the signal 366 (e.g., a ramp signal) is smaller thanthe signal 364 in magnitude, the comparator 332 generates the comparisonsignal 368 at a logic high level (e.g., 1). In yet another example, theflip-flop component 336 receives, at a “CLK” terminal, a clock signal372 generated by the oscillator 334, and receives, at an “R” terminal(e.g., a reset terminal), the comparison signal 368. If the comparisonsignal 368 is at the logic low level, the flip-flop component 336outputs a signal 370 at a logic low level (e.g., 0) to the drivingcomponent 342 through the AND gate 340 in order to turn off (e.g., open)the switch 316 in some embodiments. For example, if a rising edgeappears in the clock signal 372, the flip-flop component 336 generatesthe signal 370 at a logic high level (e.g., 1) in order to turn on(e.g., close) the switch 316. For example, the signal 366 is a rampsignal associated with a ramping period. In another example, the signal366 increases from a first magnitude to a second magnitude during afirst period in a ramping period, and decreases from the secondmagnitude to the first magnitude during a second period in the sameramping period. In yet another example, the clock signal 372 has a samefrequency as the signal 366. In yet another example, the clock signal372 has a same phase as the signal 366. In yet another example, if theclock signal 372 is at a logic high level, the signal 366 decreases inmagnitude, and if the clock signal 372 is at a logic low level, thesignal 366 increases in magnitude.

FIG. 3(B) is a simplified diagram showing a power conversion systemconfigured to operate in the critical conduction mode (CRM) (e.g., thequasi-resonant (QR) mode) according to another embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. The powerconversion system 400 includes a controller 402, a primary winding 404,a secondary winding 406, a current sensing resistor 408, an auxiliarywinding 410, three capacitors 412, 414 and 428, two resistors 418 and420, two rectifying diodes 422 and 424, and a power switch 416. Thecontroller 402 includes the current-control component 326, an amplifier430, a comparator 432, a QR detector 434, a signal generator 435, aflip-flop component 436, the demagnetization detector 338, an AND gate440, and a driving component 442. Further, the controller 402 includesterminals 444, 446, 448, and 450. For example, the power switch 416 is abipolar transistor. In another example, the power switch 416 is a fieldeffect transistor (e.g., a MOSFET).

According to one embodiment, when the power switch 416 is closed (e.g.,on), the energy is stored in the transformer including the primarywinding 404 and the secondary winding 406. For example, when the powerswitch 416 is open (e.g., off), the stored energy is released to theoutput terminal. In another example, the auxiliary winding 410 andassociated components (e.g., the resistors 418 and 420) generates afeedback signal 460 which is related to an output voltage 459 on thesecondary side. In yet another example, the demagnetization detector 338detects the feedback signal 460 and outputs a detection signal 456 whichindicates when a demagnetization process associated with the transformerincluding the primary winding 404 and the secondary winding 406 startsand when the demagnetization process ends. In yet another example, thecurrent-control component 326 receives the detection signal 456, acurrent-sensing signal 454 generated by the resistor 408 and a drivesignal 458 generated by the driving component 442, and outputs a signal462 (e.g., V_(C)) to the amplifier 430. In yet another example, theamplifier 430 and the capacitor 428 are included in an integrator whichintegrates a difference between the signal 462 and a reference signal467 and outputs a signal 464.

According to another embodiment, the comparator 432 receives the signal464 and a signal 466 generated by the signal generator 435, and outputsa comparison signal 468. For example, if the signal 466 (e.g., a rampsignal) is larger than the signal 464 in magnitude, the comparator 432generates the comparison signal 468 at a logic low level (e.g., 0). Inanother example, if the signal 466 (e.g., a ramp signal) is smaller thanthe signal 464 in magnitude, the comparator 432 generates the comparisonsignal 468 at a logic high level (e.g., 1). In yet another example, theQR detector 434 detects whether the system 400 operates in the CRM mode(e.g., the QR mode). In yet another example, the QR detector 434receives the feedback signal 460 and outputs a signal 473 to theflip-flop component 436 and the signal generator 435. In yet anotherexample, the flip-flop component 436 receives, at a “CLK” terminal, thecomparison signal 468 and receives, at an “R” terminal (e.g., a resetterminal), the signal 473.

When the comparison signal 468 is at the logic low level, the flip-flopcomponent 436 outputs a signal 470 at a logic low level (e.g., 0) to thedriving component 442 through the AND gate 440 in order to turn off(e.g., open) the switch 416 in some embodiments. For example, if thedemagnetization process is determined to be completed, the QR detector434 generates a rising edge in the signal 473. In response, theflip-flop component 436 generates the signal 472 at the logic high level(e.g., 1) in order to turn on (e.g., close) the switch 416 according tocertain embodiments. For example, the signal 466 is a ramp signalassociated with a ramping period. In yet another example, the signal 466increases in magnitude during at least a part of the ramping period. Inyet another example, the signal 473 is associated with a signal period,and includes a pulse within the signal period. In yet another example,during a cycle, the signal 466 increases in magnitude when a rising edgeof a pulse appears in the signal 473. In yet another example, the signal466 decreases to a low magnitude abruptly at the end of the cycle. Inyet another example, the ramping period of the signal 466 varies fromcycle to cycle.

As discussed above and further emphasized here, FIG. 3(A) and FIG. 3(B)are merely examples, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, the system 300as shown in FIG. 3(A) and the system 400 as shown in FIG. 3(B) can becombined into one system with overlapping components so that the systemcan operate in all of the CCM mode, the DCM mode and the CRM mode (e.g.,the QR mode).

Referring back to FIG. 3(A), in some embodiments, the bandwidth of theamplifier 330 is far smaller than the switching frequency of the powerconversion system 300, for example, if

$\frac{g_{m}}{2\pi \times C_{cmp}} < \frac{1}{10T_{s}}$where g_(m) is the transconductance associated with the amplifier 330,C_(cmp) represents the capacitance of the capacitor 328, and T_(s)represents the switching period of the system 300. As an example, if thenegative feedback loop is established, the difference between the signal362 and the reference signal 367 is integrated by the integratorincluding the amplifier 330 and the capacitor 328. In another example,the signal 364 affects the duty cycle of the drive signal 358 and thusaffects the power delivered to the output. If an average value of thesignal 362 is set approximately equal to the reference signal 367 inmagnitude as follows:

$\begin{matrix}{{V_{ref} = {{\frac{1}{T} \times {\int_{0}^{T}{V_{C}\ {\mathbb{d}t}}}} = {\frac{1}{2 \times T} \times {\int_{0}^{T}{\left( {V_{{cs}\; 1} + V_{{cs}\; 0}} \right) \times \frac{T_{dem}}{T_{s}}\ {\mathbb{d}t}}}}}}\mspace{85mu}{or}} & \left( {{Equation}\mspace{14mu} 8} \right) \\{{V_{ref} = {{\frac{1}{T} \times {\int_{0}^{T}{V_{C}\ {\mathbb{d}t}}}} = {\frac{1}{T} \times {\int_{0}^{T}{\left( V_{{cs}\;\_\frac{1}{2}T_{on}} \right) \times \frac{T_{dem}}{T_{s}}\ {\mathbb{d}t}}}}}}\mspace{79mu}{or}} & \left( {{Equation}\mspace{14mu} 9} \right) \\{V_{ref} = {{\frac{1}{K} \times {\sum\limits_{1}^{K->\infty}{V_{C}(n)}}} = {\frac{1}{K} \times {\sum\limits_{1}^{K->\infty}{{V_{{cs}\;\_\frac{1}{2}T_{on}}(n)} \times \frac{T_{dem}(n)}{T_{s}(n)}}}}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$where V_(ref) represents the reference signal 367, V_(C) represents thesignal 362, and

$V_{{cs}\;\_\frac{1}{2}T_{on}}$represents the current sensing signal 354 at the middle point of anon-time period.

For example, the following equation results from combining Equation 8(or Equation 9) with Equation 6, or combining Equation 10 with Equation7.

$\begin{matrix}{I_{out} = {N \times \frac{V_{ref}}{R_{S}}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$Thus, if the reference signal 367 is kept constant in magnitude, theoutput current 357 is kept approximately constant according to certainembodiments. In some embodiments, the above-discussed current controlscheme can be applied to the power conversion system 400 as shown inFIG. 3(B).

FIG. 4 is a simplified diagram showing certain components of thecurrent-control component 326 as part of the controller 302 or as partof the controller 402 according to certain embodiments of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. Thecurrent-control component 326 includes a sampling component 502, threeswitches 504, 508 and 510, a capacitor 506, and a NOT gate 512.

According to one embodiment, the sampling component 502 receives thedrive signal 358 (or the drive signal 458) and generates a samplingsignal 518 in order to sample the current sensing signal 354 (or thecurrent sensing signal 454). For example, the current sensing signal 354(or the current sensing signal 454) is sampled at a middle point of anon-time period. In another example, in response to the sampling signal518, the switch 504 is closed (e.g., on) or open (e.g., off). In yetanother example, when the switch 504 is closed (e.g., on), the capacitor506 is charged in response to the current sensing signal 354 (or thecurrent sensing signal 454). In yet another example, when the switch 504is open (e.g., off), the capacitor 506 (e.g., with other relatedcomponents) provides a voltage signal 524 (e.g., V_(S)) through theswitch 508 if the switch 508 is closed (e.g., on) in response to thesignal 356 (or the signal 456) during the demagnetization process. Thatis, the signal 362 (or the signal 462) is approximately equal inmagnitude to the voltage signal 524 (e.g., V_(S)) during thedemagnetization process. In yet another example, if the demagnetizationprocess has ended, the switch 510 is closed (e.g., on) in response to asignal 516 which is complementary to the signal 356 (or the signal 456),and the signal 362 (or the signal 462) is approximately equal to aground voltage 520 (e.g., 0).

FIG. 5(A) is a simplified timing diagram for the controller 302 as partof the power conversion system 300 operating in the CCM mode accordingto one embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The waveform 602 represents the drive signal 358 as afunction of time, the waveform 604 represents the current sensing signal354 as a function of time, and the waveform 606 represents the samplingsignal 518 as a function of time. The waveform 608 represents the signal356 as a function of time, the waveform 610 represents the signal 516 asa function of time, and the waveform 612 represents the signal 362(e.g., V_(C)) as a function of time. For example, a switching period,T_(s), starts at time t₃ and ends at time t₆, an on-time period, T_(on),starts at the time t₃ and ends at time t₅, and an off-time period,T_(off), starts at the time t₅ and ends at the time t₆. In anotherexample, t₃≦t₄≦t₅≦t₆.

According to one embodiment, during the on-time period, the drive signal358 keeps at a logic high level (e.g., as shown by the waveform 602),and the current sensing signal 354 increases (e.g., linearly ornon-linearly) in magnitude (e.g., as shown by the waveform 604). Forexample, the signal 356 is kept at a logic low level which indicatesthat the system 300 is not in the demagnetization process and the switch508 keeps open (e.g., off). In another example, before the middle pointof the on-time period (e.g., t₄), the sampling signal 518 is kept at thelogic high level (e.g., as shown by the waveform 604), and in responsethe switch 504 is closed (e.g., on). In yet another example, thecapacitor 506 is charged and the voltage on the capacitor 506 increasesin magnitude. In yet another example, at the middle point of the on-timeperiod (e.g., at t₄), the sampling signal 518 changes from the logichigh level to the logic low level (e.g., a falling edge shown in thewaveform 606), and the switch 504 is open (e.g., off). The voltage 524at the capacitor 506 (e.g., V_(S)) is approximately equal to a magnitude614 (e.g., the magnitude of the current sensing signal 354 at t₄) insome embodiments. For example, the magnitude 614 is approximately equalto an average of a magnitude 618 (e.g., V_(cs) _(—) ₀) of the currentsensing signal 354 at the beginning of the on-time period (e.g., at t₃)and a magnitude 620 (e.g., V_(cs) _(—) ₁) of the current sensing signal354 at the end of the on-time period (e.g., at t₅). In another example,during the on-time period, the signal 516 keeps at the logic high level(e.g., as shown by the waveform 610) and the switch 510 is closed (e.g.,on). In yet another example, the signal 362 (e.g., V_(C)) isapproximately equal to the ground voltage 520 (e.g., as shown by thewaveform 612).

According to another embodiment, at the beginning of the off-time period(e.g., t₅), the drive signal 358 changes from the logic high level tothe logic low level (e.g., as shown by the waveform 602), and thecurrent sensing signal 354 decreases to a magnitude 616 (e.g., as shownby the waveform 604). For example, the sampling signal 518 keeps at thelogic low level and the switch 504 keeps open (e.g., off). In anotherexample, the signal 356 changes from the logic low level to the logichigh level which indicates that the demagnetization process begins andthe switch 508 is closed (e.g., on). In yet another example, the signal362 (e.g., V_(C)) changes from the ground voltage to be approximatelyequal to the voltage 524 at the capacitor 506 (e.g., V_(S)) as shown bythe waveform 612. In yet another example, at the end of the off-timeperiod (e.g., at t₆), the signal 356 changes from the logic high levelto the logic low level which indicates that the demagnetization processends and thereafter a new switching cycle begins.

FIG. 5(B) is a simplified timing diagram for the controller 302 as partof the power conversion system 300 operating in the DCM mode accordingto another embodiment of the present invention. This diagram is merelyan example, which should not unduly limit the scope of the claims. Oneof ordinary skill in the art would recognize many variations,alternatives, and modifications. The waveform 632 represents the drivesignal 358 as a function of time, the waveform 634 represents thecurrent sensing signal 354 as a function of time, and the waveform 636represents the sampling signal 518 as a function of time. The waveform638 represents the signal 356 as a function of time, the waveform 640represents the signal 516 as a function of time, and the waveform 642represents the signal 362 (e.g., V_(C)) as a function of time. Forexample, a switching period, T_(s), starts at time t₇ and ends at timet₁₁, an on-time period, T_(on), starts at the time t₇ and ends at timet₉, a demagnetization time period, T_(dem), starts at the time t₉ andends at time t₁₀, and an off-time period, T_(off), starts at the time t₉and ends at the time t₁₁. In another example, t₇≦t₈≦t₉≦t₁₀≦t₁₁.

According to one embodiment, during the on-time period, the drive signal358 keeps at a logic high level (e.g., as shown by the waveform 632),and the current sensing signal 354 increases (e.g., linearly ornon-linearly) in magnitude (e.g., as shown by the waveform 634). Forexample, the signal 356 is kept at a logic low level which indicatesthat the system 300 is not in the demagnetization process and the switch508 keeps open (e.g., off). In another example, before the middle pointof the on-time period (e.g., t₈), the sampling signal 518 is kept at thelogic high level (e.g., as shown by the waveform 634), and in responsethe switch 504 is closed (e.g., on). In yet another example, thecapacitor 506 is charged and the voltage on the capacitor 506 increasesin magnitude. In yet another example, at the middle point of the on-timeperiod (e.g., at t₈), the sampling signal 518 changes from the logichigh level to the logic low level (e.g., a falling edge shown in thewaveform 636), and the switch 504 is open (e.g., off). The voltage 524at the capacitor 506 (e.g., V_(S)) is approximately equal to a magnitude644 (e.g., the magnitude of the current sensing signal 354 at t₈) insome embodiments. For example, the magnitude 644 is approximately equalto an average of a magnitude 648 (e.g., V_(cs) _(—) ₀) of the currentsensing signal 354 at the beginning of the on-time period (e.g., at t₇)and a magnitude 650 (e.g., V_(cs) _(—) ₁) of the current sensing signal354 at the end of the on-time period (e.g., at t₉). In another example,during the on-time period, the signal 516 keeps at the logic high level(e.g., as shown by the waveform 640) and the switch 510 is closed (e.g.,on). In yet another example, the signal 362 (e.g., V_(C)) isapproximately equal to the ground voltage 520 (e.g., as shown by thewaveform 642).

According to another embodiment, at the beginning of the off-time period(e.g., t₉), the drive signal 358 changes from the logic high level tothe logic low level (e.g., as shown by the waveform 632), and thecurrent sensing signal 354 decreases to a magnitude 616 (e.g., as shownby the waveform 634). For example, the sampling signal 518 keeps at thelogic low level and the switch 504 keeps open (e.g., off). In anotherexample, the signal 356 changes from the logic low level to the logichigh level which indicates that the demagnetization process begins andthe switch 508 is closed (e.g., on). In yet another example, the signal362 (e.g., V_(C)) changes from the ground voltage to be approximatelyequal to the voltage 524 at the capacitor 506 (e.g., V_(S)) as shown bythe waveform 642. In yet another example, the off-time period ends(e.g., at t₁₁) a time period after the demagnetization process hascompleted and thereafter a new switching cycle begins.

FIG. 5(C) is a simplified timing diagram for the controller 402 as partof the power conversion system 400 operating in the CRM mode (e.g., theQR mode) according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The waveform 662 representsthe drive signal 458 as a function of time, the waveform 664 representsthe current sensing signal 454 as a function of time, and the waveform666 represents the sampling signal 518 as a function of time. Thewaveform 668 represents the signal 456 as a function of time, thewaveform 670 represents the signal 516 as a function of time, and thewaveform 672 represents the signal 462 (e.g., V_(C)) as a function oftime. In some embodiments, the scheme of using the sampling signal 518to sample the current sensing signal 454 at a middle point of an on-timeperiod in the QR mode as shown in FIG. 5(C) is similar to the schemesdemonstrated in FIG. 5(A) and FIG. 5(B).

Referring to FIG. 5(A), FIG. 5(B) and FIG. 5(C), the signal 362 (or thesignal 462) can be determined as follows, for example.

$\begin{matrix}{V_{C} = {{\frac{1}{T_{s}} \times \left( {{V_{s}T_{dem}} + {0 \times T_{{dem}\;\_\; b}}} \right)} = {{V_{{cs}\;\_\frac{1}{2}T_{on}} \times \frac{T_{dem}}{T_{s}}} = {\frac{V_{{cs}\; 1} + V_{{cs}\; 0}}{2} \times \frac{T_{dem}}{T_{s}}}}}} & \left( {{Equation}\mspace{14mu} 12} \right)\end{matrix}$where V_(C) represents the signal 362 (or the signal 462), V_(s)represents the voltage signal 524, T_(s) represents the duration of aswitching period, T_(dem) represents the duration of the demagnetizationperiod, and T_(dem) _(—) _(b) represents the duration of the switchingperiod excluding the demagnetization period. In addition,

$V_{{cs}\;\_\frac{1}{2}T_{on}}$represents a magnitude of the current sensing signal 354 (or the currentsensing signal 454) at the middle point of an on-time period, V_(cs0)represents a magnitude of the current sensing signal 354 (or the currentsensing signal 454) at the beginning of the on-time period, and V_(cs1)represents a magnitude of the current sensing signal 354 (or the currentsensing signal 454) at the end of the on-time period.

FIG. 6 is a simplified diagram showing certain components of thesampling component 502 as part of the current-control component 326 ofthe controller 302 or the controller 402 according to certainembodiments of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. The sampling component 502 includes two one-shot signalgenerators 702 and 704, four switches 706, 710, 714 and 718, twocapacitors 712 and 716, a comparator 720, a NOT gate 722, two AND gates724 and 726, and a current source 708.

According to one embodiment, the one-shot signal generator 702 receivesthe drive signal 358 (or the drive signal 458) and generates a signal730 (e.g., clr1) which affects the status of the switch 714. Forexample, the one-shot signal generator 704 receives the signal 730(e.g., clr1) and outputs a signal 732 which affects the status of theswitch 710. In another example, the drive signal 358 (or the drivesignal 458) affects the status of the switch 706. In yet anotherexample, the capacitor 712 generates a voltage signal 738 when theswitch 706 is closed (e.g., on). In yet another example, when beingcharged, the capacitor 714 generates a voltage signal 736. In yetanother example, the comparator 720 receives the signal 736 and thesignal 738 and outputs a comparison signal 740 to the AND gate 724 andthe NOT gate 722 in order to generate the sampling signal 518. In yetanother example, the AND gate 726 outputs a signal 734 to affect thestatus of the switch 718.

FIG. 7 is a simplified timing diagram for the sampling component 502 aspart of the current-control component 326 of the controller 302 or thecontroller 402 according to some embodiments of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. The waveform 802represents the drive signal 358 (or the drive signal 458) as a functionof time, the waveform 804 represents the signal 730 as a function oftime, and the waveform 806 represents the signal 732 as a function oftime. The waveform 808 represents the signal 738 as a function of time,the waveform 810 represents the signal 736 as a function of time, thewaveform 812 represents the sampling signal 518 as a function of time,and the waveform 814 represents the signal 734 as a function of time.

According to one embodiment, during a time period between time t₂₀ andtime t₂₁, the drive signal 358 (or the drive signal 458) keeps at alogic high level (e.g., as shown by the waveform 802). For example, thesignal 730 (e.g., clr1) and the signal 732 (e.g., clr2) keep at a logiclow level (e.g., as shown by the waveform 804 and the waveform 806,respectively), and in response the switch 714 and the switch 710 areopen (e.g., off) respectively. In another example, the switch 706 isclosed (e.g., on) in response to the drive signal 358 (or the drivesignal 458). In yet another example, the capacitor 712 is charged andthe voltage signal 738 (e.g., V_(C1)) increases in magnitude (e.g., asshown by the waveform 808).

According to another embodiment, at the time t₂₁, the drive signal 358(or the drive signal 458) changes from the logic high level to the logiclow level (e.g., a falling edge as shown by the waveform 802) and theswitch 706 is open (e.g., off). For example, the one-shot signalgenerator 702 generates a pulse (e.g., with a pulse width between t₂₁and t₂₂) in the signal 730 (e.g., clr1) as shown by the waveform 804. Inanother example, in response, the switch 714 is closed (e.g., on) andthe capacitor 712 begins to discharge. In yet another example, thesignal 738 (e.g., V_(C1)) decreases from a magnitude 816 to a magnitude818, and the signal 736 (e.g., V_(C2)) increases to a magnitude 820. Inyet another example, the magnitude 818 is equal to the magnitude 820. Inyet another example, if the capacitance of the capacitor 712 and thecapacitance of the capacitor 716 are equal, then the magnitude 818 andthe magnitude 820 are each equal to one half of the magnitude 816.

According to yet another embodiment, at the time t₂₂, the signal 730changes to the logic low level (e.g., a falling edge as shown by thewaveform 804). For example, the switch 714 is open (e.g., off). Inanother example, the one-shot signal generator 704 generates a pulse(e.g., with a pulse width between t₂₂ and t₂₃) in the signal 732 (e.g.,clr2) as shown by the waveform 806. In another example, in response, theswitch 710 is closed (e.g., on) and the capacitor 712 is discharged. Inyet another example, the signal 738 decreases to a magnitude 822 (e.g.,0) as shown by the waveform 808), and the signal 736 keeps at themagnitude 820 (e.g., as shown by the waveform 810).

In one embodiment, at the beginning of a next switching cycle (e.g., attime t₂₄), the drive signal 358 (or the drive signal 458) changes fromthe logic low level to the logic high level (e.g., as shown by thewaveform 802). For example, in response, the switch 706 is closed (e.g.,on). In another example, the capacitor 712 is charged again and thevoltage signal 738 (e.g., V_(C1)) increases in magnitude (e.g., as shownby the waveform 808). In yet another example, if the signal 738 issmaller in magnitude than the signal 736, the comparator 720 generatesthe comparison signal 740 at a logic high level (e.g., 1). In yetanother example, if the signal 738 becomes larger in magnitude than thesignal 736 (e.g., at t₂₅, the middle point of the on-time period), thecomparator 720 changes the comparison signal 740 to a logic low level(e.g., 0). In yet another example, the AND gate 724 receives thecomparison signal 740 and the drive signal 358 (or the drive signal 458)and outputs the sampling signal 518. That is, the sampling signal 518 isequal to a logic sum of the comparison signal 740 and the drive signal358 (or the drive signal 458). In yet another example, when thecomparison signal 740 is changed to the logic low level, the AND gate726 changes the signal 734 in order to close (e.g., turn on) the switch718, and in response the capacitor 716 is discharged. In yet anotherexample, the signal 736 decreases to a low magnitude 824 (e.g., at t₂₅,the middle point of the on-time period) as shown by the waveform 810.

FIG. 8(A) is a simplified diagram showing certain components of thedemagnetization detector 338 as part of the controller 302 or as part ofthe controller 402 according to certain embodiments of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. Thedemagnetization detector 338 includes a capacitor 902, two resistors 904and 906, a comparator 908, an offset component 910, two NOT gates 912and 918, two flip-flop components 914 and 916, and an AND gate 920.

According to one embodiment, the feedback signal 360 (or the feedbacksignal 460) is received at the capacitor 902. For example, a knee pointof the feedback signal 360 (or the feedback signal 460) that indicatesthe end of the demagnetization process is detected using a differentialcircuit including the capacitor 902 and the resistors 904 and 906. Inanother example, a signal 922 is generated to be equal to adifferentiated signal related to the slope of the feedback signal 360(or the feedback signal 460) plus a direct-current (DC) offset V_(m). Inyet another example, the DC offset V_(m) is determined based on thefollowing equation.

$\begin{matrix}{V_{m} = {{AVDD} \times \frac{R_{4}}{R_{3} + R_{4}}}} & \left( {{Equation}\mspace{14mu} 13} \right)\end{matrix}$where V_(m) represents the DC offset, AVDD represents a referencevoltage 924, R₃ represents the resistance of the resistor 904, and R₄represents the resistance of the resistor 906.

According to another embodiment, the comparator 908 receives the signal922 and a threshold signal 926 (e.g., 0.1 V) generated by the offsetcomponent 910 and outputs a comparison signal 928 to the flip-flopcomponents 914 and 916. For example, the drive signal 358 (or the drivesignal 458) is processed by the NOT gate 912 in order to affect theflip-flop components 914 and 916. In another example, during thedemagnetization process, the signal 922 is no less than the thresholdsignal 926 in magnitude. In yet another example, if the signal 922becomes smaller than the threshold signal 926 in magnitude, then the endof the demagnetization process is detected. In yet another example, thecomparator 908 changes the comparison signal 928 in order to change thedetection signal 356 (or the detection signal 456).

As discussed above and further emphasized here, FIG. 8(A) is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the resistor 904 is replaced by acurrent source in some embodiments, as shown in FIG. 8(B).

FIG. 8(B) is a simplified diagram showing certain components of thedemagnetization detector 338 as part of the controller 302 or as part ofthe controller 402 according to some embodiments of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. As shown inFIG. 8(B), the demagnetization detector 338 includes a current source934, instead of the resistor 904.

According to one embodiment, the feedback signal 360 (or the feedbacksignal 460) is received at the capacitor 902. For example, a signal 936is generated, by the differential circuit including the capacitor 902and the resistors 904 and 906, to be equal to a differentiated signalrelated to the slope of the feedback signal 360 (or the feedback signal460) plus a direct-current (DC) offset V_(m). In another example, the DCoffset V_(m) is determined based on the following equation.V _(m) =I _(dc) ×R ₄  (Equation 14)where V_(m) represents the DC offset, I_(dc) represents a current 938flowing through the resistor 906, and R₄ represents the resistance ofthe resistor 906.

FIG. 9 is a simplified timing diagram for the demagnetization detector338 as part of the controller 302 operating in the DCM mode according tocertain embodiments of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The waveform 1002 represents the drive signal 358 asa function of time, the waveform 1004 represents the feedback signal 360as a function of time, the waveform 1006 represents the signal 922 as afunction of time, and the waveform 1008 represents the detection signal356 as a function of time. For example, a switching period, T_(s),starts at time t₂₆ and ends at time t₃₀, an on-time period starts at thetime t₂₆ and ends at time t₂₇, a demagnetization period, T_(dem), startsat the time t₂₇ and ends at time t₂₈. In another example,t₂₆≦t₂₇≦t₂₈≦t₂₉≦t₃₀.

According to one embodiment, during the on-time period, the drive signal358 keeps at a logic high level (e.g., as shown by the waveform 1002).For example, the flip-flop components 914 and 916 are reset. In anotherexample, when the drive signal 358 changes from the logic high level toa logic low level at the end of the on-time period (e.g., at t₂₇), thefeedback signal 360 increases to a magnitude 1010 (e.g., as shown by thewaveform 1004), and the signal 922 abruptly increases from a magnitude1012 to a magnitude 1014 (e.g., a rising edge as shown by the waveform1006) before gradually decreasing in magnitude. In yet another example,if the signal 922 becomes larger in magnitude than the threshold signal926, the comparator 908 changes the comparison signal 928. The detectionsignal 356 changes from the logic low level to the logic high level(e.g., a rising edge as shown in the waveform 1008) which indicates thedemagnetization process begins in some embodiments.

According to another embodiment, when the demagnetization process ends(e.g., at t₂₈), a knee point 1018 appears in the feedback signal 360 andthe feedback signal 360 decreases in magnitude (e.g., as shown by thewaveform 1004). For example, the signal 922 abruptly decreases from amagnitude 1016 to a magnitude 1020 (e.g., a falling edge as shown by thewaveform 1006). In another example, in response, the comparator changesthe comparison signal 928. The detection signal 356 changes from thelogic high level to the logic low level (e.g., a falling edge as shownin the waveform 1008) which indicates the demagnetization process endsin some embodiments.

FIG. 10 is a simplified timing diagram for the demagnetization detector338 as part of the controller 302 operating in the CCM mode according tosome embodiments of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The waveform 1102 represents the drive signal 358 asa function of time, the waveform 1104 represents the feedback signal 360as a function of time, the waveform 1106 represents the signal 922 as afunction of time, and the waveform 1108 represents the detection signal356 as a function of time. For example, a switching period, T_(s),starts at time t₃₂ and ends at time t₃₄, an on-time period starts at thetime t₃₂ and ends at time t₃₃, a demagnetization period, T_(dem), startsat the time t₃₃ and ends at time t₃₄. In another example, t₃₂≦t₃₃≦t₃₄.

According to one embodiment, during the on-time period, the drive signal358 keeps at a logic high level (e.g., as shown by the waveform 1102).For example, the flip-flop components 914 and 916 are reset. In anotherexample, when the drive signal 358 changes from the logic high level toa logic low level at the end of the on-time period (e.g., at t₃₃), thefeedback signal 360 increases to a magnitude 1110 (e.g., as shown by thewaveform 1104), and the signal 922 abruptly increases from a magnitude1112 to a magnitude 1114 (e.g., a rising edge as shown by the waveform1106) before gradually decreasing in magnitude. In yet another example,if the signal 922 becomes larger in magnitude than the threshold signal926, the comparator 908 changes the comparison signal 928. The detectionsignal 356 changes from the logic low level to the logic high level(e.g., a rising edge as shown in the waveform 1108) which indicates thedemagnetization process begins in some embodiments.

According to another embodiment, when the demagnetization process ends(e.g., at t₃₄), the feedback signal 360 decreases in magnitude (e.g., asshown by the waveform 1104). For example, the signal 922 abruptlydecreases from a magnitude 1116 to a magnitude 1120 (e.g., a fallingedge as shown by the waveform 1106). In another example, in response,the comparator changes the comparison signal 928. The detection signal356 changes from the logic high level to the logic low level (e.g., afalling edge as shown in the waveform 1108) which indicates thedemagnetization process ends in some embodiments. In yet anotherexample, in the CCM mode, the off-time period is approximately equal induration to the demagnetization period, and a next switching cyclestarts right after the demagnetization period ends.

As discussed above and further emphasized here, FIG. 9 and FIG. 10 aremerely examples, which should not unduly limit the scope of the claims.One of ordinary skill in the art would recognize many variations,alternatives, and modifications. For example, the schemes illustrated inFIG. 9 and FIG. 10 using the demagnetization detector 338 as part of thecontroller 302 operating in the DCM mode and in the CCM moderespectively can also be applied to the demagnetization detector 338 aspart of the controller 402 operating in the CRM mode (e.g., the QRmode).

According to another embodiment, a system controller for regulating anoutput current of a power conversion system includes a drivingcomponent, a demagnetization detector, a current-regulation componentand a signal processing component. The driving component is configuredto output a drive signal to a switch in order to affect a primarycurrent flowing through a primary winding of the power conversionsystem, the drive signal being associated with at least a switchingperiod, the switching period including an on-time period and ademagnetization period. The demagnetization detector is configured toreceive a feedback signal associated with an output voltage of the powerconversion system and generate a detection signal based on at leastinformation associated with the feedback signal, the detection signalindicating the demagnetization period. The current-regulation componentis configured to receive the drive signal, the detection signal and acurrent-sensing signal and output a current-regulation signal based onat least information associated with the drive signal, the detectionsignal, and the current sensing signal, the current-sensing signalrepresenting the primary current in magnitude. In addition, the signalprocessing component is configured to receive the current-regulationsignal and output a processed signal to the driving component in orderto generate the drive signal. For example, the system controller isimplemented according to at least FIG. 3(A) and/or FIG. 3(B).

According to another embodiment, a system controller for regulating anoutput current of a power conversion system includes a drivingcomponent, a current-regulation component, an amplifier and acomparator. The driving component is configured to output a drive signalto a switch in order to affect a primary current flowing through aprimary winding of the power conversion system, the drive signal beingassociated with at least a switching period, the switching periodincluding an on-time period and a demagnetization period. Thecurrent-regulation controller is configured to receive the drive signal,a detection signal and a current-sensing signal and output a firstsignal based on at least information associated with the drive signal,the detection signal, and the current sensing signal, the detectionsignal indicating the demagnetization period, the current-sensing signalrepresenting the primary current in magnitude. The amplifier isconfigured to receive the first signal and a reference signal andgenerate, with at least a capacitor, an amplified signal based on atleast information associated with the first signal and the referencesignal. The comparator configured to receive the amplified signal and aramp signal and generate a comparison signal based on at leastinformation associated with the amplified signal and the ramp signal,the ramp signal being associated with at least a ramping period. Thedriving component is further configured to receive the comparison signaland a second signal associated with at least a signal period, processinformation associated with the comparison signal and the second signal,and generate the drive signal based on at least information associatedwith the comparison signal and the second signal. For example, thesystem controller is implemented according to at least FIG. 3(A) and/orFIG. 3(B).

According to yet another embodiment, a system controller for detecting ademagnetization period associated with a power conversion systemincludes a differentiation component, a comparator, and a detectioncomponent. The differentiation component is configured to receive afeedback signal associated with an output signal of the power conversionsystem and output a processed signal based on at least informationassociated with the feedback signal. The comparator is configured toreceive at least the processed signal and generate a comparison signalbased on at least information associated with the processed signal. Thedetection component is configured to receive at least the comparisonsignal and output a detection signal based on at least informationassociated with the comparison signal. The differentiation componentincludes a capacitor, a resistor and a current source. The capacitorincludes a first capacitor terminal and a second capacitor terminal, thefirst capacitor terminal being configured to receive the feedbacksignal. The resistor includes a first resistor terminal and a secondresistor terminal, the first resistor terminal being configured tooutput the processed signal, the second resistor terminal being biasedto a predetermined voltage. The current source includes a firstcomponent terminal and a second component terminal, the first componentterminal being coupled to the second capacitor terminal and the firstresistor terminal. For example, the system controller is implementedaccording to at least FIG. 3(A), FIG. 3(B), FIG. 8(A), FIG. 8(B), FIG.9, and/or FIG. 10.

In one embodiment, a method for regulating an output current of a powerconversion system includes outputting a drive signal to a switch inorder to affect a primary current flowing through a primary winding ofthe power conversion system, the drive signal being associated with atleast a switching period, the switching period including an on-timeperiod and a demagnetization period, receiving a feedback signalassociated with an output voltage of the power conversion system, andprocessing information associated with the feedback signal. The methodfurther includes generating a detection signal based on at leastinformation associated with the feedback signal, the detection signalindicating the demagnetization period, receiving the drive signal, thedetection signal and a current-sensing signal, the current-sensingsignal representing the primary current in magnitude, and processinginformation associated with the drive signal, the detection signal andthe current-sensing signal. In addition, the method includes outputtinga current-regulation signal based on at least information associatedwith the drive signal, the detection signal, and the current sensingsignal, receiving the current-regulation signal, processing informationassociated with the current-regulation signal, and outputting aprocessed signal to the driving component in order to generate the drivesignal. For example, the method is implemented according to at leastFIG. 3(A) and/or FIG. 3(B).

In another embodiment, a method for regulating an output current of apower conversion system includes outputting a drive signal to a switchin order to affect a primary current flowing through a primary windingof the power conversion system, the drive signal being associated withat least a switching period, the switching period including an on-timeperiod and a demagnetization period, receiving the drive signal, adetection signal and a current-sensing signal, the detection signalindicating the demagnetization period, the current-sensing signalrepresenting the primary current in magnitude, and processinginformation associated with the drive signal, the detection signal andthe current-sensing signal. The method further includes outputting afirst signal based on at least information associated with the drivesignal, the detection signal, and the current sensing signal, receivingthe first signal and a reference signal, and processing informationassociated with the first signal and the reference signal. In addition,the method includes generating, with at least a capacitor, an amplifiedsignal based on at least information associated with the first signaland the reference signal, receiving the amplified signal and a rampsignal, the ramp signal being associated with at least a ramping period,and processing information associated with the amplified signal and theramp signal. Furthermore, the method includes generating a comparisonsignal based on at least information associated with the amplifiedsignal and the ramp signal, receiving the comparison signal and a secondsignal associated with at least a signal period, processing informationassociated with the comparison signal and the second signal, andgenerating the drive signal based on at least information associatedwith the comparison signal and the second signal. For example, themethod is implemented according to at least FIG. 3(A) and/or FIG. 3(B).

In yet another embodiment, a method for detecting a demagnetizationperiod associated with a power conversion system includes receiving, byat least a capacitor, a feedback signal associated with an output signalof the power conversion system, the capacitor including a firstcapacitor terminal and a second capacitor terminal, the first capacitorterminal receiving the feedback signal, the second capacitor terminalbeing coupled to a first component terminal of a current source, thecurrent source further including a second component terminal, providinga current, by the current source, to at least a resistor including afirst resistor terminal and a second resistor terminal, the firstresistor terminal being coupled to the first component terminal, thesecond resistor terminal being biased to a predetermined voltage, andprocessing information associated with the feedback signal and thecurrent. The method further includes outputting, by at least the firstresistor terminal, a processed signal based on at least informationassociated with the feedback signal and the current, receiving at leastthe processed signal and a reference signal, and processing informationassociated with the processed signal and the reference signal. Inaddition, the method includes generating a comparison signal based on atleast information associated with the processed signal and the referencesignal, receiving at least the comparison signal, and generating adetection signal based on at least information associated with thecomparison signal. For example, the method is implemented according toat least FIG. 3(A), FIG. 3(B), FIG. 8(A), FIG. 8(B), FIG. 9, and/or FIG.10.

For example, some or all components of various embodiments of thepresent invention each are, individually and/or in combination with atleast another component, implemented using one or more softwarecomponents, one or more hardware components, and/or one or morecombinations of software and hardware components. In another example,some or all components of various embodiments of the present inventioneach are, individually and/or in combination with at least anothercomponent, implemented in one or more circuits, such as one or moreanalog circuits and/or one or more digital circuits. In yet anotherexample, various embodiments and/or examples of the present inventioncan be combined.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

What is claimed is:
 1. A system controller for regulating an outputcurrent of a power conversion system, the system controller comprising:a driving component configured to output a drive signal to a switch inorder to affect a primary current flowing through a primary winding ofthe power conversion system, the drive signal being associated with atleast a switching period, the switching period including an on-timeperiod and a demagnetization period; a demagnetization detectorconfigured to receive a feedback signal associated with an outputvoltage of the power conversion system and generate a detection signalbased on at least information associated with the feedback signal, thedetection signal indicating the demagnetization period; acurrent-regulation component configured to receive the drive signal, thedetection signal and a current-sensing signal and output acurrent-regulation signal based on at least information associated withthe drive signal, the detection signal, and the current-sensing signal,the current-sensing signal representing the primary current inmagnitude; and a signal processing component configured to receive thecurrent-regulation signal and output a processed signal to the drivingcomponent in order to generate the drive signal; wherein the drivingcomponent includes: a flip-flop component configured to receive theprocessed signal and a first signal and output a second signal based onat least information associated with the processed signal and the firstsignal; and a driver configured to receive the second signal and outputthe drive signal based on at least information associated with thesecond signal.
 2. The system controller of claim 1 wherein thedemagnetization detector is further configured to: if the powerconversion system operates in the demagnetization period, generate thedetection signal at a first logic level; and if the power conversionsystem does not operate in the demagnetization period, generate thedetection signal at a second logic level, the first logic level beingdifferent from the second logic level.
 3. The system controller of claim1 wherein the detection signal represents whether the demagnetizationperiod has started and whether the demagnetization period has ended. 4.The system controller of claim 1 wherein the driver includes an AND gateconfigured to receive the second signal and the first signal.
 5. Thesystem controller of claim 1 wherein the signal processing componentincludes: an amplifier configured to receive the current-regulationsignal and a reference signal and generate, with at least a capacitor,an amplified signal based on at least information associated with thecurrent-regulation signal and the reference signal; and a comparatorconfigured to receive the amplified signal and generate the processedsignal based on at least information associated with the amplifiedsignal.
 6. The system controller of claim 5 wherein the signalprocessing component further includes: a signal generator configured tooutput the first signal to the flip-flop component and output a rampsignal to the comparator, the first signal being associated with a clockfrequency corresponding to a clock period, the ramp signal beingassociated with a ramping frequency corresponding to a ramping period.7. The system controller of claim 6 wherein: the ramping frequency isequal to the clock frequency; and the ramping period is equal to theclock period in duration.
 8. The system controller of claim 5 whereinthe signal processing component further includes: a quasi-resonancedetector configured to receive the feedback signal and output the firstsignal to the flip-flop component; and a signal generator configured toreceive the first signal and output a ramp signal to the comparator, theramp signal being associated with at least a ramping period.
 9. Thesystem controller of claim 8 wherein the quasi-resonance detector isfurther configured to, if the feedback signal indicates thedemagnetization period has ended, change the first signal from a firstlogic level to a second logic level.
 10. The system controller of claim9 wherein the signal generator is further configured to, if the firstsignal changes from the first logic level to the second logic level,increase the ramp signal in magnitude.
 11. The system controller ofclaim 8 wherein the signal generator is further configured to decreasethe ramp signal in magnitude at the end of the ramping period.
 12. Thesystem controller of claim 8 wherein the ramping period varies induration over time.
 13. A system controller for regulating an outputcurrent of a power conversion system, the system controller comprising:a driving component configured to output a drive signal to a switch inorder to affect a primary current flowing through a primary winding ofthe power conversion system, the drive signal being associated with atleast a switching period, the switching period including an on-timeperiod and a demagnetization period; a demagnetization detectorconfigured to receive a feedback signal associated with an outputvoltage of the power conversion system and generate a detection signalbased on at least information associated with the feedback signal, thedetection signal indicating the demagnetization period; acurrent-regulation component configured to receive the drive signal, thedetection signal and a current-sensing signal and output acurrent-regulation signal based on at least information associated withthe drive signal, the detection signal, and the current-sensing signal,the current-sensing signal representing the primary current inmagnitude; and a signal processing component configured to receive thecurrent-regulation signal and output a processed signal to the drivingcomponent in order to generate the drive signal; wherein thedemagnetization detector includes: a differentiation componentconfigured to receive the feedback signal and output a first signalbased on at least information associated with the feedback signal; acomparator configured to receive at least the first signal and generatea comparison signal; and a detection component configured to receive thecomparison signal and the drive signal and output the detection signalbased on at least information associated with the comparison signal andthe drive signal.
 14. The system controller of claim 13 wherein thedifferentiation component includes: a capacitor including a firstcapacitor terminal and a second capacitor terminal; a first resistorincluding a first resistor terminal and a second resistor terminal; anda second resistor including a third resistor terminal and a fourthresistor terminal; wherein: the first resistor terminal is biased to afirst voltage; the second resistor terminal is connected to the thirdresistor terminal; the second capacitor terminal is connected to thesecond resistor terminal; and the fourth resistor terminal is biased toa second voltage; wherein: the first capacitor terminal is configured toreceive the feedback signal; and the second resistor terminal isconfigured to output the first signal.
 15. A system controller forregulating an output current of a power conversion system, the systemcontroller comprising: a driving component configured to output a drivesignal to a switch in order to affect a primary current flowing througha primary winding of the power conversion system, the drive signal beingassociated with at least a switching period, the switching periodincluding an on-time period and a demagnetization period; ademagnetization detector configured to receive a feedback signalassociated with an output voltage of the power conversion system andgenerate a detection signal based on at least information associatedwith the feedback signal, the detection signal indicating thedemagnetization period; a current-regulation component configured toreceive the drive signal, the detection signal and a current-sensingsignal and output a current-regulation signal based on at leastinformation associated with the drive signal, the detection signal, andthe current-sensing signal, the current-sensing signal representing theprimary current in magnitude; and a signal processing componentconfigured to receive the current-regulation signal and output aprocessed signal to the driving component in order to generate the drivesignal; wherein the current-regulation component includes: asample-and-hold component configured to receive the drive signal, samplethe current-sensing signal at a middle point of the on-time period, andhold the sampled current-sensing signal; and a signal-selectioncomponent configured to receive the detection signal and output thecurrent-regulation signal; wherein the signal-selection component isfurther configured to, if the detection signal indicates that the powerconversion system operates in the demagnetization period, output theheld sampled current-sensing signal; and if the detection signalindicates that the power conversion system does not operate in thedemagnetization period, output a ground voltage signal.
 16. The systemcontroller of claim 15 wherein the sample-and-hold component includes: atiming component configured to receive the drive signal and output asampling signal; a switch configured to be open or closed in response tothe sampling signal; and a capacitor coupled to the switch; wherein: thetiming component is further configured to output the sampling signal ata logic high level during a first half of the on-time period and outputthe sampling signal at a logic low level during a second half of theon-time period; the switch is further configured to be open if thesampling signal is at the logic low level and be closed if the samplingsignal is at the logic high level; and the capacitor is configured toreceive the current-sensing signal through the switch if the switch isopen.
 17. The system controller of claim 15 wherein the signal-selectioncomponent includes: a first switch coupled to the sample-and-holdcomponent and configured to be closed if the detection signal indicatesthe power conversion system operates in the demagnetization period andbe open if the detection signal indicates the power conversion systemdoes not operate in the demagnetization period; and a second switchconfigured to be open if the detection signal indicates the powerconversion system operates in the demagnetization period and be closedif the detection signal indicates the power conversion system does notoperate in the demagnetization period; wherein the second switch isfurther configured to receive the ground voltage signal.
 18. A systemcontroller for regulating an output current of a power conversionsystem, the system controller comprising: a driving component configuredto output a drive signal to a switch in order to affect a primarycurrent flowing through a primary winding of the power conversionsystem, the drive signal being associated with at least a switchingperiod, the switching period including an on-time period and ademagnetization period; a current-regulation controller configured toreceive the drive signal, a detection signal and a current-sensingsignal and output a first signal based on at least informationassociated with the drive signal, the detection signal, and thecurrent-sensing signal, the detection signal indicating thedemagnetization period, the current-sensing signal representing theprimary current in magnitude; an amplifier configured to receive thefirst signal and a reference signal and generate, with at least acapacitor, an amplified signal based on at least information associatedwith the first signal and the reference signal; and a comparatorconfigured to receive the amplified signal and a ramp signal andgenerate a comparison signal based on at least information associatedwith the amplified signal and the ramp signal, the ramp signal beingassociated with at least a ramping period; wherein the driving componentis further configured to: receive the comparison signal and a secondsignal associated with at least a signal period; process informationassociated with the comparison signal and the second signal; andgenerate the drive signal based on at least information associated withthe comparison signal and the second signal.
 19. The system controllerof claim 18 wherein: the second signal includes a pulse within thesignal period; and the ramp signal increases in magnitude during atleast a part of the ramping period.
 20. The system controller of claim18 wherein: the ramping period includes a first period and a secondperiod; during the first period, the ramp signal increases from a firstmagnitude to a second magnitude; and during the second period, the rampsignal decreases from the second magnitude to the first magnitude. 21.The system controller of claim 18 wherein: the ramp signal is associatedwith a ramping frequency corresponding to the ramping period; the secondsignal is associated with a signal frequency corresponding to the signalperiod; the ramping frequency is equal to the signal frequency; and theramping period is equal to the signal period.
 22. The system controllerof claim 18 wherein the second signal changes from a first logic levelto a second logic level if the demagnetization period has ended.
 23. Thesystem controller of claim 22 wherein, if the second signal changes fromthe first logic level to the second logic level, the ramp signalincreases in magnitude.
 24. The system controller of claim 23 whereinthe ramp signal decreases in magnitude at the end of the ramping period.25. The system controller of claim 18 wherein the ramping period variesin duration over time.
 26. The system controller of claim 18 wherein thedriving component includes: a flip-flop component configured to receivethe comparison signal and the second signal and output a third signalbased on at least information associated with the comparison signal andthe second signal; and a driver configured to receive the third signaland output the drive signal based on at least information associatedwith the third signal.
 27. The system controller of claim 26, andfurther comprising: a signal generator configured to output the secondsignal to the flip-flop component and output the ramp signal to thecomparator, the second signal being associated with a signal frequencycorresponding to the signal period, the ramp signal being associatedwith a ramping frequency corresponding to the ramping period.
 28. Thesystem controller of claim 27 wherein: the ramping frequency is equal tothe signal frequency; and the ramping period is equal to the signalperiod in duration.
 29. The system controller of claim 26, and furthercomprising: a quasi-resonance detector configured to receive a feedbacksignal and output the second signal to the flip-flop component, thesecond signal indicating whether the power conversion system operates ina quasi-resonant mode; and a signal generator configured to receive thesecond signal and output the ramp signal to the comparator.
 30. Thesystem controller of claim 18 wherein the detection signal representswhether the demagnetization period has started and whether thedemagnetization period has ended.
 31. The system controller of claim 18wherein the amplifier includes a transconductance amplifier.
 32. Thesystem controller of claim 18 wherein the current-regulation controllerincludes: a sample-and-hold component configured to receive the drivesignal, sample the current-sensing signal at a middle point of theon-time period, and hold the sampled current-sensing signal; and asignal-selection component configured to receive the detection signaland output the first signal; wherein the signal-selection component isfurther configured to, if the detection signal indicates that the powerconversion system operates in the demagnetization period, output theheld sampled current-sensing signal; and if the detection signalindicates that the power conversion system does not operate in thedemagnetization period, output a ground voltage signal.
 33. The systemcontroller of claim 32 wherein the sample-and-hold component includes: atiming component configured to receive the drive signal and output asampling signal; a switch configured to be open or closed in response tothe sampling signal; and a capacitor coupled to the switch; wherein: thetiming component is further configured to output the sampling signal ata logic high level during a first half of the on-time period and outputthe sampling signal at a logic low level during a second half of theon-time period; the switch is further configured to be open if thesampling signal is at the logic low level and be closed if the samplingsignal is at the logic high level; and the capacitor is configured toreceive the current-sensing signal through the switch if the switch isopen.
 34. The system controller of claim 32 wherein the signal-selectioncomponent includes: a first switch coupled to the sample-and-holdcomponent and configured to be closed if the detection signal indicatesthe power conversion system operates in the demagnetization period andbe open if the detection signal indicates the power conversion systemdoes not operate in the demagnetization period; and a second switchconfigured to be open if the detection signal indicates the powerconversion system operates in the demagnetization period and be closedif the detection signal indicates the power conversion system does notoperate in the demagnetization period; wherein the second switch isfurther configured to receive the ground voltage signal.
 35. A systemcontroller for detecting a demagnetization period associated with apower conversion system, the system controller comprising: adifferentiation component configured to receive a feedback signalassociated with an output signal of the power conversion system andoutput a processed signal based on at least information associated withthe feedback signal; a comparator configured to receive at least theprocessed signal and generate a comparison signal based on at leastinformation associated with the processed signal; and a detectioncomponent configured to receive at least the comparison signal andoutput a detection signal based on at least information associated withthe comparison signal; wherein the differentiation component includes: acapacitor including a first capacitor terminal and a second capacitorterminal, the first capacitor terminal being configured to receive thefeedback signal; a resistor including a first resistor terminal and asecond resistor terminal, the first resistor terminal being configuredto output the processed signal, the second resistor terminal beingbiased to a predetermined voltage; and a current source including afirst component terminal and a second component terminal, the firstcomponent terminal being coupled to the second capacitor terminal andthe first resistor terminal.
 36. The system controller of claim 35wherein the detection component includes: a first flip-flop componentconfigured to receive at least the comparison signal and output a firstsignal based on at least information associated with the comparisonsignal; a second flip-flop component configured to receive at least thecomparison signal and output a second signal based on at leastinformation associated with the comparison signal; and a logic componentconfigured to receive the first signal and the second signal and outputthe detection signal based on at least information associated with thefirst signal and the second signal.
 37. The system controller of claim35, and further comprising a driving component configured to output adrive signal to a switch in order to affect a primary current flowingthrough a primary winding of the power conversion system, the drivesignal being associated with a switching frequency corresponding to aswitching period, the switching period including an on-time period and ademagnetization period; wherein the driving component is furtherconfigured to output the drive signal to the detection component.
 38. Amethod for regulating an output current of a power conversion system,the method comprising: outputting a drive signal to a switch in order toaffect a primary current flowing through a primary winding of the powerconversion system, the drive signal being associated with at least aswitching period, the switching period including an on-time period and ademagnetization period; receiving a feedback signal associated with anoutput voltage of the power conversion system; processing informationassociated with the feedback signal; generating a detection signal basedon at least information associated with the feedback signal, thedetection signal indicating the demagnetization period; receiving thedrive signal, the detection signal and a current-sensing signal, thecurrent-sensing signal representing the primary current in magnitude;processing information associated with the drive signal, the detectionsignal and the current-sensing signal; outputting a current-regulationsignal based on at least information associated with the drive signal,the detection signal, and the current-sensing signal; receiving thecurrent-regulation signal; processing information associated with thecurrent-regulation signal; and outputting a processed signal to thedriving component in order to generate the drive signal.
 39. A methodfor regulating an output current of a power conversion system, themethod comprising: outputting a drive signal to a switch in order toaffect a primary current flowing through a primary winding of the powerconversion system, the drive signal being associated with at least aswitching period, the switching period including an on-time period and ademagnetization period; receiving the drive signal, a detection signaland a current-sensing signal, the detection signal indicating thedemagnetization period, the current-sensing signal representing theprimary current in magnitude; processing information associated with thedrive signal, the detection signal and the current-sensing signal;outputting a first signal based on at least information associated withthe drive signal, the detection signal, and the current-sensing signal;receiving the first signal and a reference signal; processinginformation associated with the first signal and the reference signal;generating, with at least a capacitor, an amplified signal based on atleast information associated with the first signal and the referencesignal; receiving the amplified signal and a ramp signal, the rampsignal being associated with at least a ramping period; processinginformation associated with the amplified signal and the ramp signal;generating a comparison signal based on at least information associatedwith the amplified signal and the ramp signal; receiving the comparisonsignal and a second signal associated with at least a signal period;processing information associated with the comparison signal and thesecond signal; and generating the drive signal based on at leastinformation associated with the comparison signal and the second signal.40. A method for detecting a demagnetization period associated with apower conversion system, the method comprising: receiving, by at least acapacitor, a feedback signal associated with an output signal of thepower conversion system, the capacitor including a first capacitorterminal and a second capacitor terminal, the first capacitor terminalreceiving the feedback signal, the second capacitor terminal beingcoupled to a first component terminal of a current source, the currentsource further including a second component terminal; providing acurrent, by the current source, to at least a resistor including a firstresistor terminal and a second resistor terminal, the first resistorterminal being coupled to the first component terminal, the secondresistor terminal being biased to a predetermined voltage; processinginformation associated with the feedback signal and the current;outputting, by at least the first resistor terminal, a processed signalbased on at least information associated with the feedback signal andthe current; receiving at least the processed signal and a referencesignal; processing information associated with the processed signal andthe reference signal; generating a comparison signal based on at leastinformation associated with the processed signal and the referencesignal; receiving at least the comparison signal; and generating adetection signal based on at least information associated with thecomparison signal.
 41. The system controller of claim 13 wherein thedetection signal represents whether the demagnetization period hasstarted and whether the demagnetization period has ended.
 42. The systemcontroller of claim 13 wherein the demagnetization detector is furtherconfigured to: if the power conversion system operates in thedemagnetization period, generate the detection signal at a first logiclevel; and if the power conversion system does not operate in thedemagnetization period, generate the detection signal at a second logiclevel, the first logic level being different from the second logiclevel.
 43. The system controller of claim 15 wherein the detectionsignal represents whether the demagnetization period has started andwhether the demagnetization period has ended.
 44. The system controllerof claim 15 wherein the demagnetization detector is further configuredto: if the power conversion system operates in the demagnetizationperiod, generate the detection signal at a first logic level; and if thepower conversion system does not operate in the demagnetization period,generate the detection signal at a second logic level, the first logiclevel being different from the second logic level.
 45. The method ofclaim 38 wherein the detection signal represents whether thedemagnetization period has started and whether the demagnetizationperiod has ended.
 46. The method of claim 38 wherein the generating adetection signal based on at least information associated with thefeedback signal includes: if the power conversion system operates in thedemagnetization period, generating the detection signal at a first logiclevel; and if the power conversion system does not operate in thedemagnetization period, generating the detection signal at a secondlogic level, the first logic level being different from the second logiclevel.